Condensed Consolidated Statements of Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Shares of Beneficial Interest	Additional Paid-In Capital	Net Income Less Than Accumulated Dividends	Accumulated Other Comprehensive Loss	Noncontrolling Interests
Beginning Balance at Dec. 31, 2017	$ 1,809,842	$ 3,897	$ 1,772,066	$ (137,065)	$ (6,170)	$ 177,114
Increase (Decrease) in Equity [Roll Forward]
Net Income	148,969			146,824		2,145
Shares repurchased and cancelled	(8,108)	(9)	(8,099)
Shares issued under benefit plans, net	5,346	7	5,339
Dividends paid - common shares	(50,836)			(50,836)
Distributions to noncontrolling interests	(884)					(884)
Contributions from noncontrolling interests	41					41
Other comprehensive loss (income)	(1,983)				(1,983)
Ending Balance at Mar. 31, 2018	1,906,343	3,895	1,769,306	(35,580)	(9,694)	178,416
Beginning Balance at Dec. 31, 2018	1,750,699	3,893	1,766,993	(186,431)	(10,549)	176,793
Increase (Decrease) in Equity [Roll Forward]
Net Income	51,254			49,666		1,588
Shares issued under benefit plans, net	8,151	10	8,141
Dividends paid - common shares	(50,816)			(50,816)
Distributions to noncontrolling interests	(1,572)					(1,572)
Contributions from noncontrolling interests	326					326
Other comprehensive loss (income)	69				69
Other, net	2,323		1,955			368
Ending Balance at Mar. 31, 2019	$ 1,760,434	$ 3,903	$ 1,777,089	$ (187,581)	$ (10,480)	$ 177,503